The present invention relates to a display apparatus employing EL (Electro Luminescence) elements, organic EL elements, or other light-emitting type display elements (light-emitting elements), and a drive method therefor.
Light-emitting (or self-luminous) elements have the characteristic that the luminance of light emitted from them is proportional to the amount of current flowing through them, making it possible to provide a gray scale display by controlling the amount of current flowing in the elements. A plurality of such light-emitting elements may be arranged so as to form a display apparatus.
Displays using active matrix light-emitting elements are advantageous over those using simple matrix light-emitting elements in the luminance of the screen and power consumption. Each pixel of a display using active matrix light-emitting elements, however, requires a TFT (Thin Film Transistor) element capable of performing accurate V-I conversion from signal (voltage) level variations to current variations.
One method for providing a gray scale display without using such TFT elements, disclosed in JP-A-2000-235370, is to set a gray scale level for each pixel using pulse width modulation according to an input signal during each frame period.
Another problem with displays using light-emitting elements arises when the light-emitting elements are used for a long period of time. Light-emitting elements degrade over time, leading to a reduction in the luminance of their light. U.S. Pat. No. 6,291,942 (JP-A-2001-13903) discloses a technique for compensating for variations in the luminance of a light-emitting element due to its degradation over time.
JP-A-2000-330517 discloses a technique for causing an organic EL to emit light at a predetermined luminance level on average. This technique measures the magnitude of the current flowing in the organic EL to measure the amount of charge injected into it, and controls this amount by cutting off the supply of the gate voltage to the drive transistor when the total amount of the charge has reached a predetermined value.
JP-A-2000-221945 discloses a technique for increasing the number of gray scale levels which can be displayed without increasing the number of the data bits. This technique controls the voltage applied to the panel based on an average of the luminance levels of the video signals for each field such that, for example, the peak luminance level is increased when the average luminance level is low and the peak luminance level is decreased when the average luminance level is high.
The technique disclosed in the above U.S. Pat. No. 6,291,942 (JP-A-2001-13903), however, only compensates for a reduction in the luminance of light emitted from a degraded light-emitting element by changing the voltage applied to the element or adjusting the signal pulse width in order to cause the element to emit light at a proper luminance level. Therefore, this technique in no way delays degradation of the light-emitting element itself.
The techniques disclosed in the above JP-A-2000-235370, JP-A-2000-330517, and JP-A-2000-221945 also do not delay degradation of light-emitting elements.
A light-emitting element degrades more quickly with increasing current density of the element, that is, increasing luminance of light emitted from it. However, simply decreasing the display luminance of light-emitting elements to delay their degradation lowers the display quality of the display apparatus. Light-emitting elements have the property that their voltage-current density characteristic changes with temperature. Since the luminance of light emitted from a light-emitting element is proportional to the amount of current flowing in the element, as described above, the luminance of light emitted from the light-emitting element changes with temperature. This means that the luminance of light emitted from a light-emitting element may excessively increase due to temperature variation, which may accelerate the degradation. Conversely, if the luminance of light emitted from the light-emitting element is reduced due to temperature variation, the image quality will be deteriorated.
The present invention is intended to provide a display apparatus and method for increasing peak luminance of a display having a high gray scale level (for example, white) while reducing a rise in the luminance of a display having a low gray scale level (for example, black).